C#读写锁ReaderWriterLockSlim

https://blog.csdn.net/m0_37879526/article/details/109962340    我在CSDN上的博客,这里再补充一个  

ReaderWriterLockSlim 读写锁

其实 ReaderWriterLockSlim  与  ReaderWriterLock  比较像

1.某个线程进入读取模式时,此时其他线程依然能进入读取模式。假设此时一个线程要进入写入模式,那么他不得不被阻塞。直到读取模式退出为止。

2.采用读写锁,则多个线程可以同时读取该对象,只有等到对象被写入锁占用的时候,才会阻塞。  写入锁占用时,读写锁都会被堵塞

3.如果某个线程进入了写入模式,那么其他线程无论是要写入还是读取,都是会被阻塞的。

进入写入/读取模式有2种方法:
EnterReadLock尝试进入写入模式锁定状态。
TryEnterReadLock(Int32) 尝试进入读取模式锁定状态,可以选择整数超时时间。
EnterWriteLock 尝试进入写入模式锁定状态。
TryEnterWriteLock(Int32) 尝试进入写入模式锁定状态,可以选择超时时间。

退出写入/读取模式有2种方法:
ExitReadLock 减少读取模式的递归计数,并在生成的计数为 0(零)时退出读取模式。
ExitWriteLock 减少写入模式的递归计数,并在生成的计数为 0(零)时退出写入模式。

 

public class Program
  {
    static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
    static void Main(string[] args)
    {
      Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
      t_read1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
      Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
      t_read2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
      Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
      t_write1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
    }
    static public void ReadSomething()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterReadLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(5000);//模拟读取信息
        Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitReadLock();
        Console.WriteLine("{0} Thread ID {1} ExitReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
    static public void WriteSomething()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterWriteLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(10000);//模拟写入信息
        Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitWriteLock();
        Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
  }

  

 

 可以看到3号线程和4号线程能够同时进入读模式,而5号线程过了5秒钟后(即3,4号线程退出读锁后),才能进入写模式。
把上述代码修改一下,先开启2个写模式的线程,然后在开启读模式线程,代码如下:

 static void Main(string[] args)
    {
      Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
      t_write1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
      Thread t_write2 = new Thread(new ThreadStart(WriteSomething));
      t_write2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write2.GetHashCode());
      Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
      t_read1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
      Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
      t_read2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
    }

  

 

 3号线程和4号线程都要进入写模式,但是3号线程先占用写入锁,因此4号线程不得不等了10s后才进入。5号线程和6号线程需要占用读取锁,因此等4号线程退出写入锁后才能继续下去。

 

 

 

TryEnterReadLock和TryEnterWriteLock可以设置一个超时时间,运行到这句话的时候,线程会阻塞在此,如果此时能占用锁,那么返回true,如果到超时时间还未占用锁,那么返回false,放弃锁的占用,直接继续执行下面的代码。

 

 

 

static void Main(string[] args)
        {
            Console.OutputEncoding = Encoding.UTF8;
            //Console.WriteLine(Thread.CurrentThread.GetHashCode() + "begin");
            //callMethod();
            //Console.WriteLine(Thread.CurrentThread.GetHashCode() + "end");

            Task[] tasks = new Task[5];
            tasks[0] = Task.Factory.StartNew(WriteBook);
            tasks[1] = Task.Factory.StartNew(ReadBook);
            tasks[2] = Task.Factory.StartNew(WriteBook);
            tasks[3] = Task.Factory.StartNew(ReadBook);
            tasks[4] = Task.Factory.StartNew(WriteBook);

            //var task=Task.Factory.StartNew(ReadBook);
            //task.Start();
            //Task.WaitAll(tasks);

            Console.ReadKey();
        }
        static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
        // 测试 try超时
        
        public static void ReadBook()
        {
            Console.WriteLine(Thread.CurrentThread.GetHashCode()+"  Begin Read");
            
            if (rwl.TryEnterReadLock(3000))
            {
                try
                {
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "  Enter Read");
                    Thread.Sleep(5000);
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Read 获取读取锁成功");
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Read XXXXXXX");
                }
                finally
                {
                    rwl.ExitReadLock();
                }
            }
            else
            {
                Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Read 获取写入锁失败");
            }

            Console.WriteLine(Thread.CurrentThread.GetHashCode()+"  End Read");
        }
        public static void WriteBook()
        {
            Console.WriteLine(Thread.CurrentThread.GetHashCode() + "  Begin Write");

            if (rwl.TryEnterWriteLock(3000))
            {
                try
                {
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "  Enter Write");
                    Thread.Sleep(1000);
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Write 获取写入锁成功");
                    Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Write XXXXXXX");
                }
                finally
                {
                    rwl.ExitWriteLock();
                }
            }
            else
            {
                Console.WriteLine(Thread.CurrentThread.GetHashCode() + "Write 获取写入锁失败");
            }

            Console.WriteLine(Thread.CurrentThread.GetHashCode() + "  End Write");
        }

  

 

 

EnterUpgradeableReadLock
ReaderWriterLockSlim类提供了可升级读模式,这种方式和读模式的区别在于它还有通过调用 EnterWriteLock 或 TryEnterWriteLock 方法升级为写入模式。 因为每次只能有一个线程处于可升级模式。进入可升级模式的线程,不会影响读取模式的线程,即当一个线程进入可升级模式,任意数量线程可以同时进入读取模式,不会阻塞。如果有多个线程已经在等待获取写入锁,那么运行EnterUpgradeableReadLock将会阻塞,直到那些线程超时或者退出写入锁。

static public void UpgradeableRead()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterUpgradeableReadLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} doing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        rwl.EnterWriteLock();
        try
        {
          Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
          Thread.Sleep(10000);//模拟写入信息
          Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        }
        finally
        {
          rwl.ExitWriteLock();
          Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        }
        Thread.Sleep(10000);//模拟读取信息
        Console.WriteLine("{0} Thread ID {1} doing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitUpgradeableReadLock();
        Console.WriteLine("{0} Thread ID {1} ExitUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }

  

有一个官方的例子

private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
private Dictionary<int, string> innerCache = new Dictionary<int, string>();

public AddOrUpdateStatus AddOrUpdate(int key, string value)
{
    cacheLock.EnterUpgradeableReadLock();
    try
    {
        string result = null;
        if (innerCache.TryGetValue(key, out result))
        {
            if (result == value)
            {
                return AddOrUpdateStatus.Unchanged;
            }
            else
            {
                cacheLock.EnterWriteLock();
                try
                {
                    innerCache[key] = value;
                }
                finally
                {
                    cacheLock.ExitWriteLock();
                }
                return AddOrUpdateStatus.Updated;
            }
        }
        else
        {
            cacheLock.EnterWriteLock();
            try
            {
                innerCache.Add(key, value);
            }
            finally
            {
                cacheLock.ExitWriteLock();
            }
            return AddOrUpdateStatus.Added;
        }
    }
    finally
    {
        cacheLock.ExitUpgradeableReadLock();
    }
}

  

递归

使用允许递归锁的构造方法(即new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion))构造ReaderWriterLockSlim实例

如何判断代码逻辑里会触发递归锁呢,抓住两个要点“同一线程(即ManageThreadId一样)”和“同一个锁”,在线程池里运行的时候,由于可以重用工作线程,很容易触发递归锁的问题,比如这样:线程池线程1获取到UpgradeableReadLock锁后由于后面代码里有长时等待任务,调度器可能调度线程1运行队列里的另一个同样代码的任务,这时由于代码一样线程1再次申请获取UpgradeableReadLock锁(因为上一次他已经获取到该UpgradeableReadLock锁了),就会报错了

posted @ 2021-09-08 19:10  zq爱生活爱代码  阅读(201)  评论(0编辑  收藏  举报